The invention relates to permanently magnetized annular structures useful primarily for stators in dynamo electric machines, and more particularly to stators that are molded from synthetic plastic materials which serve as a matrix for embedded particles of magnetically anisotropic ferrite materials that have been magnetically oriented during the molding of the plastic material. The stator structures contemplated by the invention are preferably molded by centrifugal casting procedures, and the invention further relates to methods of manufacturing such stators and to certain devices that are useful in carrying out the centrifugal casting procedures.
The use of particulate anisotropic ferrite materials in the manufacture of permanently magnetized structures is well advanced in the art. Such materials have a hexagonal crystal structure and exhibit the phenomena when subjected to a highly intense magnetic field of retaining a higher magnetic charge in one direction in the crystal structure than in others. The anisotropic ferrite materials have the advantage over many of the metal and metal alloy types of permanent magnets, such as those made from alnico, in that they exhibit a higher coercive force and hence are especially suitable for many applications where vibrations and other demagnetizing phenomena make it unsuitable to use the cast metal magnetic structures.
One of the problems which has confronted the industrial use of the anisotropic ferrite materials resides in the attainment of high density ferrite structures that provide a high remanence in the finished magnet. One of the approaches which has been taken to overcome this problem in the manufacture of permanently magnetized ferrite stators used in motors and other dynamo electric machines has been to mold the particulate ferrite materials under high pressures and to thereafter sinter the molded structures so as to attain the structural stability needed for the contemplated application. Such procedures usually provide for a suitable magnetic orientation of the particles during the molding procedures and the subsequent subjection of the sintered product to an intense magnetizing field along the path of orientation to develop the desired remanence in the cast structure. The sintering procedures have several disadvantages, among which may be mentioned the fact that resulting structures are brittle, are frequently warped as a result of the sintering process and thereby unsuitable for the intended application, and frequently provide a magnetic field with a nonuniform flux density.
Attempts have also been made to manufacture annular magnetic structures from the anisotropic materials by procedures which avoid the sintering steps. Such attempts have involved the casting in an appropriately shaped mold of a suitable plastic material containing a suspension of the anisotropic ferrite particulate materials. During such procedures, and while the plastic material is fluid or in a semi-fluid state, the ferrite particles are magnetically oriented in accord with the desired polar arrangement in the stator structure. Such procedures, although being useful, suffer from the difficulties associated with the attainment of high particle densities and thus the high remanence values in the cast stator products. Experience has shown that the high viscosities which are encountered in handling a deformable mass of synthetic material containing high concentrations (e.g. greater than 80%) of the particulate anisotropic ferrite materials still require high mechanical pressures to be exerted during the casting procedures in order for the materials to take the shape of the mold or die, and that the high viscosities also tend to deter mobility of the particles as they are magnetically oriented during the casting procedures.